1. Field of the Invention
The present invention relates to a fuel supply system for an internal combustion engine.
2. Description of the Related Art
Attempts have long been made to use hydrogen as a fuel for an internal combustion engine, but such attempts were abandoned. Recently however, interest in these attempts has been revived due to the finding that hydrogen produces a low amount of toxic emissions, which is advantageous from the view point of protecting the environment, and due to the urgent need for the development of new fuel resource because of the looming shortage of fossil fuels in the near future. In an automobile using hydrogen as a fuel, a fuel tank having a high pressure resistance and a cylindrical shape, in which liquid hydrogen is stored, is usually employed. Such a high pressure type tank, however, is disadvantageous from the view point of easy handling, and therefore, a new construction of a fuel tank has been proposed, in place of the high pressure type, wherein a hydrogen absorbing and storing alloy, such as Ti-Fe, is charged in a tank as fine pellets or powders thereof, and gaseous hydrogen is absorbed by the Ti-Fe pellets to provide a metal hydride stored in the pellets. Heating the pellets causes the hydrogen gas to be separated therefrom. The hydrogen gas can thus be supplied to an internal combustion engine. The handling of such a system using Ti-Fe pellets is relatively easy, compared with that of a high pressure hydrogen tank, because a relatively large amount of high density hydrogen can be stored under a relatively small pressure.
Japanese Unexamined Patent Publication No. 61-220009 discloses a tank to be charged with the metal hydride pellets. This tank is equipped with a system for maintaining a predetermined constant pressure inside the metal hydride tank regardless of a change in a load on the internal combustion engine, and the system is constructed by a pressure sensor for detecting a pressure of the hydrogen gas in the tank, and an electric control device for calculating a pressure change in accordance with a lapse of time, i.e., a pressure gradient. A flow control valve is further provided and is responsive to a calculated pressure gradient for controlling an amount of hot engine cooling water fed from the engine to the tank, for heating the metal hydride, to thus control the amount of hydrogen gas separated from the pellets stored in the tank.
In Japanese Unexamined Patent Publication No. 1-216024, the metal hydride tank is provided with a temperature sensor, in addition to the pressure sensor. Furthermore, in addition to a main metal hydride tank, an auxiliary tank is provided and arranged in parallel to the main tank, and a switching valve is located in a pipe to the tanks, for controlling a selective introduction of the hot engine water to the tanks. A warning device is further provided for sounding an alarm when the amount of hydrogen remaining in the tank becomes too small, to thus prevent a situation such that the fuel supply is abruptly stopped while the vehicle is running. This prior art is provided with a plurality of pressure sensors arranged in gas passageways running from the tank to the internal combustion engine, a pressure control device, and a flow control device. The pressure control device and the flow control device are operated in accordance with the pressure values detected by the pressure sensors, to thus obtain a required control of the pressure and flow amount of the hydrogen gas.
In a prior art hydrogen engine provided with a metal hydride tank, as also seen from the above-mentioned first and second prior arts, the control of the amount of the hydrogen gas is executed by a predetermined heating or cooling of the metal hydride material, and the obtained hydrogen gas is introduced, via a pressure regulator device, to a carburetor or to fuel injection valves of the internal combustion engine. The pressure regulator mounted in a fuel supply conduit between the fuel tank and the carburetor or fuel injection valves is provided for maintaining a predetermined constant pressure of the hydrogen gas in the conduit. Such a control of the hydrogen pressure by the regulator is essential for obtaining a desired amount of hydrogen by controlling the duration of the time for which the fuel injection valves are open, without actually detecting the amount of hydrogen gas. Such a fuel supply system, however, suffers from the following drawbacks.
The regulator in the hydrogen supply conduit connecting the metal hydride tank with the fuel injectors can suffer from a large pressure loss across the regulator, compared with a gasoline engine, and this makes it difficult for the hydrogen engine to quickly obtain an amount of hydrogen gas required. Namely, hydrogen has a small molecular weight, and therefore, with respect to the same amount of air introduced into the internal combustion engine, a large volumetric amount of the hydrogen gas must be introduced when compared with a high density hydrocarbon fuel such as gasoline. Such a large volumetric amount of the hydrogen gas creates a large pressure loss across the regulator when the gas is passed therethrough, and therefore, upon an acceleration of the engine, the amount of hydrogen required by the engine is increased but the large pressure loss makes it difficult for the necessary amount of hydrogen gas to be quickly supplied to the engine, and thus the performance of the engine is degraded.